[Land-speed] Rear Axle Question

[23weldon]

 There is a comment on my spread sheet that applies:
"It's called slip percent here but it's really a "fudge factor" with several 
components including actual wheel slippage, errors in measuring actual 
effective tire diameter, tachometer error, slip in automatic transmissions 
and whatever I haven't thought of.  ........ (speed of light effects, 
curvature of the Earth in the traps)
While there may be some personal reason to know your exact speed on a 
landspeed course the only speed that is really important is the official 
timed speed.  The purpose of my spread sheet is not to tell you the speed 
you went.  You get that from the timing slip.  The purpose of the spread 
sheet is not to tell you how fast the car will go.  You already know that, 
obviously with some degree of error before your first run and fairly exactly 
after the run.
The purpose is to tell you first what the slip percent is and then what will 
be the gross effect of a change in tire diameter,  gear ratio or engine RPM. 
It will be unique to the actual timed run of the actual car.  BTW, the only 
tire diameter you can practically measure is what you get when the car is 
sitting still. Exactness about this measurement is probably not all that 
important as long as you are consistent about how you do it. Salt conditions 
and engine tune are much bigger variables.
By noting the time of day and other observable conditions you get a data set 
that further helps you in understanding the slip numbers for your car and 
the course.  This information is obviously important for setting the car up 
for a qualifying or record setting run.  But it is also important for early 
licensing in a really fast car to control the car and keep it from getting 
over the maximum allowed speed.   Which spot on the tach should be there in 
your frontal cortex as you deal with the things you need to do to keep the 
damn thing under control.  Example: an A or B license run in a really hot 
roadster will likely be made under full engine tune conditions, especially 
if the car/engine is fairly new.  (I'm thinking the thousand horsepower 
small block you can almost buy in a crate.)
Ed Weldon

----- Original Message ----- 
From: "Kirkwood" <saltfever at comcast.net>
To: "land-speed" <land-speed at autox.team.net>
Sent: Wednesday, November 05, 2014 9:18 PM
> There are essentially two speed formulas. The one Ed gave uses the tire
> DIAMETER and you divide by 336. The other formula uses the tire RADIUS and
> you divide by 168. Since a radius is half of a diameter it stands to 
> reason
> the 336 constant would be divided by 2. So what is the difference ......